This invention relates generally to automatic test equipment (ATE) for the testing of electronic circuits, and more particularly the invention relates to an ultra-high-speed digital test system using electro-optic signal sampling.
Conventional automatic test equipment (ATE) systems and ATE technology are inadequate to test high-speed integrated circuits including silicon bipolar emitter-coupled logic circuits, gallium arsenide circuits, and high-speed CMOS/NMOS circuits The present state of the art in test-system technology has maximum data rates running up to 200 MHz. The difficulties in raising this test rate limit are largely concentrated on the receiver technology, and measurement and signal routing which degrade the signals to the point of causing inaccuracies in the measurement system.
A high-speed conventional LSI/VLSI test system is a complex electro-mechanical assembly. The system must meet stringent requirements in throughput, pin count, voltage and time accuracy, and must be general-purpose enough to accommodate the manufacturer's present and future device types. The test system must also perform DC and AC parametrics, have flexible real-time branching "on the fly," and support many different waveform formats. Lastly, the test system must have a comprehensive software package to assist the manufacturer in developing his own test programs.
In spite of all of the above, test systems have until recently kept pace with the device requirements. At the 10 MHz clock rate and 48-64 pin requirement of over ten years ago, the manufacturers of test systems were able to cope. Even at 20 MHz and up to 120 pins (e.g. the Fairchild Sentry 20), cost-effective test systems were built. However, the push from 20 MHz to 40 or 50 MHz has been more difficult. The predominant reasons behind the difficulties in manufacturing a faster test system are technical in nature.
Accommodating all of the features of a general test system has meant substantial capacitance seen by the pin of a device under test (DUT). Present VLSI testers have pin capacitances from a low of 22 pF to 100 pF or more. This capacitance is difficult to reduce and can cause major accuracy problems in testing MOS circuits. For high-speed testing, pin capacitance is a major consideration and should be kept below 5 pF.
High pin count has caused modern ATE systems to have a large number of complex electronic assemblies placed near the DUT. A conventional test head has the necessary resources to inject complex tri-state test waveforms to the DUT, power the device, and measure its output waveforms. Because of the amount of electronics required, DUT pin-to-receiver distances are forced to be as long as 50 cm through a series of connectors. High-speed signal fidelity suffers which reduces the available bandwidth of the test system. Changing device impedances during switching also degrades total measurement performance irrespective of any controlled impedance paths to the receiver.
To eliminate reflection problems, the receiver must be placed in close proximity to the DUT within a distance corresponding to a quarter wavelength of the highest frequency of interest For a receiver bandwidth of 5 GHz into 50 ohms, for example, the maximum pin-to-receiver distance is approximately 0.5 cm, creating a very difficult mechanical and cooling problem.
Timing accuracy affects the quality of a test and is therefore a major consideration in any test system. For current ATE, timing accuracy (pin-to-pin skew) is sub-nanosecond. For example, two conventional test systems have an overall timing accuracy of 900 psec and 700 psec. Considering the large number of pins and the distances involved, this is an amazing accomplishment. For high-speed testing, however, the overall timing accuracy must be below 100 psec to maintain reasonable tester correlation and tester stability. Current test systems fall short of reaching gigahertz test requirements.
Electro-optic sampling techniques utilizing the Pockels effect have been proposed for use in ATE systems. Named after Friedrich Pockels, a German physicist who studied the phenomenon in the late 1800's, this effect is a fundamental physical inter-action between light and an electric field across an appropriate crystal. The effect causes the polarity of the light passing through the crystal to rotate proportionally to the intensity of the electric field impressed across the crystal. In other words, the crystal serves as a polarization modulator of light.
Gunn U.S. Pat. No. 3,614,451 describes a sampling system which utilizes electro-optic techniques for sampling an electrical signal. Gunn proposes the use of a travelling wave Pockels cell in which an electrical signal is propagated through a microstrip placed on a crystal exhibiting either a linear or longitudinal electro-optic effect. Light pulses are propagated through the crystal in the same general direction as the propagated electrical signal, and, due to electrically-induced birefringence, the state of polarization of the light pulses is altered according to the electrical field intensity to which the electro-optic crystal is subjected by that portion of the electrical signal travelling coincidentally along the transmission line structure.
As disclosed by Yarif, Quantum Electronics, John Wiley & Sons, 1967, 1975, the modulation of optical radiation in both longitudinal and transverse modes of modulation are well known. Valdmanis et al. U.S. Pat. No. 4,446,425 discloses an electro-optic sampling system similar to the system disclosed by Gunn but in which the modulation of the optical radiation occurs in a transverse mode. The Valdmanis et al. electro-optic sampling system utilizes a travelling-wave Pockels cell in which the light pulses propagate across the cell in a direction transverse to the propagation of the electrical signal along the travelling-wave Pockels cell.
Bloom et al. U.S. Pat. No. 4,681,449 disclose a high-speed testing circuit in which a travelling-wave Pockels cell comprising gallium arsenide is employed. Bloom et al. utilize a dual-wave picosecond optical source to simultaneously excite a gallium arsenide photodiode and to measure the birefringence induced by the gallium arsenide transmission line by the electro-optic effect.
E.G.& G. Princeton Applied Research has introduced a 350 GHz sampling oscilloscope which utilizes electro-optic sampling as previously disclosed by Valdmanis et al.